The green roof is becoming a more common occurrence in Australian development as the industry looks for improved sustainability and better amenity solutions. But could a new breed of green roof, gaining popularity in the US over the last couple of years, provide even better outcomes?
In simple terms, a green roof is partially or fully covered with vegetation and a growing medium, planted over a waterproof membrane. Sod roofs have been used in Scandinavia for centuries.
They provide a number of benefits, including thermal and sound insulation, amenity space, biodiversity, reduced water runoff and improved water runoff quality.
There are four key types, depending on soil depth and maintenance options.
An ‘Extensive’ green roof is a self-sustaining ecological landscape; a ‘Biodiverse’ one will have a locally sourced growing medium, naturally vegetated; ‘Semi-intensive’ is a hybrid of extensive and intensive; and ‘Intensive’ is equivalent to a garden or park.
Extensive Green Roof
These require only a shallow substrate of 60 to 200 millimetres.
Vegetation might include moss, herbs, grasses, wild flowers (sedum or similar), which are planted during installation.
They are virtually self-sustaining and require minimal maintenance, with annual weeding and the annual application of slow release fertilizer typically sufficing.
Their low cost and low upkeep are upsides, but they tend to be used for inaccessible roofs and do not provide amenity space.
The weight consideration for structural engineers is 60 to 150 kilograms per square metre.
Biodiverse Green Roof
Industrial brownfield sites can be important ecosystems, supporting rare species of plants, animals and invertebrates. The redevelopment of these sites threatens these habitats. Similar to extensive green roofs, these biodiverse options help to mitigate this loss.
The growing medium is typically recycled material sourced from or around the site. Vegetation is then left to grow naturally, replacing lost habitat, and coverage develops much more slowly. Additions such as “insect hotels” can also be incorporated but these require specific expertise and knowledge to design successfully.
Their self-sustaining nature means little to no maintenance is required.
Semi-intensive Green Roof
These require a moderate substrate of 120 to 250 millimetres.
Vegetation might include herbs, grasses, wild flowers (Sedum or similar) and shrubs, like the extensive green roof, planted during installation.
As they tend to be used for accessible and highly visible roofs and provide amenity space, they require more maintenance. This includes regular weeding, regular application of fertilizer and shrub pruning
The weight consideration for structural engineers is 120 to 200 kilograms per square metre.
Intensive Green Roof
This really is a true roof garden requiring a deep substrate of 150 to 400 millimetres.
Vegetation might include lawn, shrubs and trees but again vegetation can still be planted during installation.
Unsurprisingly, these high quality amenity spaces mean higher costs and greater upkeep requirements.
The significant maintenance includes regular weeding and lawn mowing, regular application of fertilizer and shrub and tree pruning
They are also more of a challenge for structural engineers requiring a weight consideration of 180 to 500 kilograms per square metre.
The blue roof is a fairly new source of alternative energy and conservation.
It is explicitly designed to capture and slowly release rainwater in order to slow the rate of runoff and reduce the potential for related flooding.
The stored rainfall can be reused for irrigation purposes, as makeup water for cooling, or in recreational contexts.
Blue roofs are often incorporated with any type of green roof to provide hydration to vegetation, although they can also be provided under paving.
They can be combined with light-colored roofing materials to further mitigate urban heat island effects.
They reduce or eliminate the need for underground storage, which in turn educes excavated material and underground construction cost and time.
They are a viable retrofit for existing buildings, although structural capacity needs to be carefully considered and the maintenance required is equal to that of a conventional roof.
During and after rainfall, weirs at roof drain inlets restrict the flow rate of the captured water, creating temporary ponding before gradually releasing stormwater to the municipal sewer system. Most weirs allow for adjustment of the flow rate.
The ratio of captured runoff is typically designed to closely mimic the pre-construction hydrology of the site. A blue roof acts as a temporary sponge, replacing the prior capacity of undeveloped, pervious ground to absorb rainwater.
Blue roofs are gaining particular popularity in New York, where rooftops have increasingly been repurposed from a utilitarian function into places that can be used for the use of the public and for the benefit of the environment.
Rooftops collectively constitute almost 20 per cent of the entire area of New York City. The city’s policy document, PlaNYC, states that “rooftops represent our last big frontier.” The city provides a Green Roof Tax Abatement from City property taxes of $4.50 per square foot of green roof, up to $100,000 and administrators believe the utilization of blue and green roofs could save New York City $2.4 billion over 20 years; without the roofs, taxpayers could end up spending $6.8 billion repairing constantly flooded treatment plants.
Blue roofs complement long and flat roofing styles, and have wide gutters with a sturdy watertight liner. This design works especially well in highly urbanized areas, like New York City, where less space is available for on-site stormwater detention. As cities around Australia and the globe become increasingly dense, blue roofs are certain to be become more and more common.